Mosquito and Tick-Borne Diseases

West Nile Encephalitis

Mosquito and Tick-Borne Diseases

Disease Diction

Encephalitis is swelling of the brain. It can be caused by viruses and bacteria, including those transmitted to people by mosquitoes.

West Nile encephalitis is another viral infection that is transmitted from mosquitoes to people. The first documented case of the disease in the Western Hemisphere was in the United States in 1999. Although West Nile has caused tremendous worry, it has not caused that many infections. This disease has also been found in Africa, West Africa, Eastern Europe, and the Middle East.

In 1999, there were 62 severe cases of West Nile in the United States. Seven people died. In 2000, there were 21 cases and two deaths in New York City. There are no worldwide statistics available on the incidence of this disease.

In the United States, the disease is most likely to strike in the late summer and early fall. In warmer climates, the disease may strike at any time. The highest number of cases of West Nile have been in Florida, where mosquitoes have the opportunity to breed year-round.

From Birds, to Mosquitoes, to the Human Nervous System

West Nile infection occurs when an infected mosquito bites a person. The mosquito gets the virus from feeding on infected birds. The virus lives in the mosquitoes' salivary glands and travels through the human bloodstream to enter the brain and interfere with the central nervous system. Essentially, it causes swelling of the brain tissue.

West Nile virus expansion in North America. (Courtesy United States Department of Defense)

Less than one percent of those infected will develop severe illness. Three to five percent of those with severe illness will die.

Most West Nile infections are mild. Symptoms include fever, headache, and body aches. Sometimes there is a skin rash and swollen glands. The incubation period is three to fifteen days.

Dealing with West Nile

West Nile is diagnosed through patient history and blood tests.

Like dengue and yellow fever, there is no specific treatment for West Nile infection. In severe cases, patients are hospitalized, given intravenous fluids, and put on a ventilator to help them breathe. Sometimes antibiotics are given to prevent secondary infection.

There is a vaccination available for horses, but no vaccine yet for people.

Mosquito Management Is Key to Prevention

Infectious Knowledge

Unlike many infections that are species-specific, West Nile has been seen in birds, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

As with other bug-borne diseases, good mosquito management to track growth in mosquito populations helps to prevent West Nile infections. This also includes using insecticides in high incidence areas. Disease surveillance is also an important component of a good prevention strategy.

Antigen Alert

The appearance of dead birds may be a warning that West Nile is present in an area. Public health officials now test birds for the presence of the virus.

Staying indoors at dawn and dust, wearing long-sleeved shirts and long pants, and using bug repellent containing DEET or permethrin can help, too. Bug repellent should be sprayed on clothes and skin.

Potent Fact

There is no evidence of the infection being contagious between vertebrate species. In other words, people can't get West Nile from a horse or a cat.

Army Spc. Clark Dutterer, a preventive medicine specialist at the U.S. Army Center for Health Promotion and Preventive Medicine at Aberdeen Proving Ground, Maryland, sets a light trap to catch adult mosquitoes in the Maryland woods. The mosquitoes are then tested to see if they are carrying certain diseases. The trap was supplied by the Centers for Disease Control and Prevention. (Photo by Sgt. 1st Class Kathleen T. Rhem, USA. Photo courtesy United States Department of Defense)